The present invention relates to a receiver for being mounted on an automobile telephone or similar mobile station included in a cellular mobile radio communication system. More particularly, the present invention is concerned with a receiver capable of enhancing the reception sensitivity characteristic in locations where the reception field strength is low, or the intermodulation distortion characteristic in locations where the field sensitivity is high and intermodulation distortions occur.
A cellular mobile radio communication system divides a service area into minute cells and locates a base station in each of the cells. This kind of communication system allows the base stations to communicate with mobile stations such as automobile telephones while promoting effective use of frequencies. A conventional receiver for such an application has an antenna duplexer for passing, among received input waves (having various wavelenght) coming in through an antenna, only a desired wave (having a desired wavelength). A high frequency amplifier amplifies the received wave while a high frequency filter attenuates the frequencies of the received wave lying outside of a particular frequency band. A first frequency converter has a first mixer and a first local oscillator and converts the received wave passed through the high frequency filter to a first intermediate frequency (IF). The first IF is applied to an IF filter. A second frequency converter has a second mixer and a second local oscillator and converts the first IF passed through the IF to a second IF. The second IF is fed to an IF amplifier and then to a demodulator. The demodulated signal from the demodulator is amplified by a low frequency amplifier and then outputted via a loudspeaker as voice. A field strength detector monitors the reception field strength by converting the field strength of the received input wave from the antenna to a voltage. A SAT (Supervisory Audible Tone) signal detector is responsive to a SAT signal being fed from a base station for determining whether or not the mobile station is in communication with the base station.
The above-described receiver has an excellent reception sensitivity characteristic since the high frequency amplifier amplifies the received wave and since the first mixer converts it to the first IF with a high gain. However, when the desired received wave is accompanied by interference waves, the intermodulation distortion is aggravated, degrades the intermodulation characteristic.
Some different efforts have heretofore been made to reduce the intermodulation distortions and thereby improve the intermodulation characteristic. For example, the high frequency amplifier may be omitted to thereby lower the level of the interference waves entering the first mixer, or the gain of the first mixer may be reduced. However, the problem with these implementations is that the high frequency gain of the receiver is lowered, which degrades the receive sensitivity due to the omission of the high frequency amplifier or due to the lowered gain of the first mixer.